twitter/bootstrap — pull request #3992 by philfreo (2012-07-04 06:21:52+00:00)
Title: Adding a way to have buttons-radio but where a clicked button can be un-selected

Simple change: `buttons-radio-optional` behaves like `buttons-radio` except it allows the user to deselect a chosen button.

Basically, if you have two buttons in a `buttons-radio` group, by using `buttons-radio-optional` instead you could have a 3rd state of neither button selected.